The demand for higher energy density power sources for electronics devices is continually increasing as these devices shrink in size. This demand is being increasingly met using recently developed lithium primary and rechargeable battery systems. The commercial success of such systems, in part, relies on the availability of suitable non-aqueous electrolytes. High voltage (&gt;3 V) and high rate (of order of 1C rate) lithium battery systems require an electrolyte that is stable over a wide range in potential and has relatively high ionic conductivity. Only a handful of lithium salts currently exist that are suitable candidates for use in such electrolytes. The salt LiPF.sub.6 is one of these, and has received wide attention since it is also low in toxicity and stable in solution. Recently, Sony Energy Tec Inc. has produced the first commercially available lithium ion type battery in which the electrolyte contains LiPF.sub.6 salt dissolved in a mixture of diethylcarbonate (DEC) and propylene carbonate (PC) solvents.
LiPF.sub.6 based electrolytes are generally prepared by dissolving solid LiPF.sub.6, or a complex thereof, in the desired electrolyte solvents.
The heat of solution can be significant, and generally some means of temperature control must be used to prevent overheating and subsequent decomposition of the salt. LiPF.sub.6 itself can be prepared using one of several methods described in the literature. Many of these methods however result in a product with levels of impurities that are unsuitable for use in battery applications. For example, LiPF.sub.6 can be made by reacting BF.sub.3 with LiF and an excess of P.sub.2 O.sub.5, but the product always contains LiF.
Adequately pure LiPF.sub.6 has recently been produced by reacting PF.sub.5 with LiF in liquid HF. Such a method is used to manufacture LiPF.sub.6 for commercial purposes. However, this method involves using the hazardous compounds HF and PF.sub.5, and requires complex manufacturing equipment. Also, the product contains residual HF at a level of order of 200 to 300 ppm. The product is nonetheless suitable for use in lithium ion batteries but is relatively expensive.
U.S. Pat. No. 3,654,330 discloses an alternate method of preparing pure LiPF.sub.6 from a Li(CH.sub.3 CN).sub.4 PF.sub.6 precursor. CH.sub.3 CN is required rather than HF using this method.
LiPF.sub.6, however, even in its pure form is reported to be somewhat unstable, decomposing to PF.sub.5 and LiF. Improper storage and handling of the solid accelerates this decomposition. The consequent limited shelf life and more stringent storage and handling requirements are all undesirable features associated with use of said salt. These problems are overcome to some extent by preparing solid complexes of LiPF.sub.6. In the aforementioned patent (U.S. Pat. No. 3,654,330), the Li(CH.sub.3 CN).sub.4 PF.sub.6 precursor is a complex of LiPF.sub.6 and CH.sub.3 CN. Complexing the salt in this way stabilizes the salt against decomposition.
Similarly U.S. Pat. No. 4,880,714 discloses the preparation of a complex of LiPF.sub.6 and an ether that is stable against decomposition. Said preparation involves reacting a salt of the form (XH).sup.+ PF.sub.6.sup.-, wherein (XH).sup.+ is a cation comprising an adduct of a proton (H.sup.+) and a Lewis base (X), with a lithium base of the form LiY in an ether based solvent. A solid complex of LiPF.sub.6 and the ether is obtained by recrystallization and isolation steps. A battery electrolyte containing said ether can be prepared thereafter simply by dissolving the complex in additional appropriate solvents. However, unless said ether is desirable or at least acceptable in the electrolyte, such simple preparation is not possible. Unfortunately, ethers can be undesirable in many battery applications for a variety of reasons but especially because of possible adverse effects on battery safety. Similarly, CH.sub.3 CN is rarely considered desirable for use in electrolytes for commercial lithium batteries as it is not adequately stable against lithium.
Thus, state-of-the-art methods for preparing LiPF.sub.6 based electrolytes, in particular electrolytes for lithium ion type batteries, generally involve recrystallization and isolation steps. The use of hazardous compounds is often involved and salt Stability is a concern.